The successful implementation of future sub 0.1 .mu.m semiconductor designs into marketable chips depends to a large degree on verifying the manufacturability of optimally designed device structures. Key design criteria for modeling and the realization of optimal performance are the tailored distributions (shapes) and concentration gradients of implanted dopant atoms within the active region of the device and their confirmation by analytical methods. Dopant concentrations are expected to be in the 10.sup.18 to 10.sup.20 /cm.sup.3 range, which corresponds to an average dopant atom separation of 10 nm (nanometer) to 2.2 nm, respectively. Furthermore, the dopant implanted regions are expected to be delineated or defined over dimensions of the order of 5 to 20 nm in the device, which places unrealizable demands on analytical methods to establish the existence of such intended structures. In essence, the verification of such highly delineated structures requires a technique that is sensitive to a broad range of dopant distributions and that exhibits a spatial resolution of 5 nm or less.
Historically, quasi-quantitative etching and staining methods sensitive to dopant concentrations have been used, followed by examinations with optical or electron microscopes. These methods exhibit unacceptable resolutions for modern device technology.
More recently, two methods have been proposed that are based on an atomic force microscope (AFM) fitted with an electrically conducting tip. The first method correlates the changes in resistance (called the spreading resistance) measured by directly contacting the semiconductor with the AFM tip to determine the doping concentration (P. De Wolf et al. J. Vac. Sci. Technol. A13, 1699 (1995)). Such a method claims a resolution of 50 nm.
The second AFM-based method measures the capacitance under the AFM tip, which is then correlated to the dopant concentration (Y. Huang, C. C. Williams and J. Slinkman, Appl. Phys. Lett. 66, 344 (1995)). This instrument, referred as the Scanning Capacitance Microscope (SCM), has been commercialized by Digital Instruments, Inc., Santa Barbara, Calif. The instrument claims a resolution of about 10 nm.
Aside from the limited resolution, which is dependent on the tip size and shape, as well as dopant concentrations, both of the above AFM methods suffer from excessive tip wear, are computationally intensive in extracting dopant concentrations, require calibration standards and are chemically insensitive to the dopant type and polarity.
Accordingly, it is an object of the present invention to provide an AFM-based microscope and method that provide improved resolution based on intrinsic physical arguments, instead of on dopant concentrations.
It is a further object of the present invention to provide an AFM-based microscope and method that can identify individual atoms to measure a broad range of concentrations.
Another object of the present invention is to provide an AFM-based microscope and method that is predominantly sensitive to dopant atoms within 1 to 5 nm of a substrate surface.
It is a further object of the present invention to provide a self calibrating AFM-based microscope which measures isolated dopant atoms.
It is also an object of the present invention to provide an AFM-based microscope that is chemically specific through appropriate choices of the exciting light frequency.
Another object of the present invention is to provide an AFM-based microscope and method that minimizes tip wear.